1) Field of the Invention
The present invention relates to a direct type lighting system, an image display apparatus using the same and a light diffusion plate used therefore. Especially, the present invention relates to a direct type lighting system, an image display apparatus using the same and a light diffusion plate used therefore which are preferably used for large electric signboards and liquid crystal display devices required high luminance and luminance uniformity.
2) Description of the Related Art
As for example of the lighting system for the image display apparatus, there are two types of lighting systems, one of which is an edge light type that the light from a light source is directed to the front direction by an optical waveguide and uniformly distributed through a diffusion sheet, and the other of which is an direct type that a light source is disposed behind the lighting plane and the light is uniformly distributed through a light diffusion plate.
As for the direct type, the thickness is inclined to increase because the light source is disposed behind the body. Alternatively, as for the edge light type, the light source is disposed on the edge. Therefore, the edge light type is more appropriate for cellular phones and mobile computers which are required the thinness so that it has been popularly used.
Meanwhile, recently the demand for a large-screen and high luminance display is increased in the market of televisions and computer screens mainly. Along with the display is enlarged, the edge light type can not provide sufficient luminance because the ratio of the length of the peripheral portion in which the light source can be disposed to the display area is reduced so that it is not supplied with sufficient quantities of light.
In consideration of the above-described problem, the method of improving the efficiency of using light by disposing a plurality of films to increase luminance on the surface illuminant was proposed. (cf. Patent document 1)
However, the luminance increasing film comes expensive and the large number of films is required so that it is not always appropriate in view of productivity and reducing the thickness of the body. Additionally, the edge light type has another problem that the weight of the optical waveguide is increased along with enlarging the display. Thus, recently it is difficult for the edge light type to meet the requirements of the marketplace such as the large-screen and high luminance display.
Thus for large-screen televisions and computer screens which are not strictly required to reduce the thickness of the body than cellular phones and mobile computers, the direct type is paid attention. In the direct type, the efficiency of using light emitted from the light source, that is to say, the percentage of the light flux emitted from the light-emitting surface among light flux emitted from the light source is high, and the number of the light source can be freely increased.
In other words, the quantity of light can be freely increased so that the required high luminance is easily obtained and the luminance and the luminance uniformity are not reduced even if the display is enlarged. Further, the weight can be reduced because the optical waveguide to guide light to the front is not required.
As for other lighting systems, the electric signboard for example is mainly the direct type because its configuration is simple and high luminance is easily obtained without using the luminance increasing film.
However, the directly-under system should resolve the unique problems such as eliminating lamp image, thinning the body and low power consumption. Especially, the lamp image significantly appears as ununifomity of luminance than the edge light type. Additionally, the thickness of the display is disadvantage than the edge light system. Therefore, in order to reduce the thickness of the display, the configuration should be changed such that the member in thickness direction is thinned and the distance between which the members are disposed is narrowed. Thus it is further difficult to resolve the problem of the lamp image. The conventional means used by the edge light type, i.e. the diffusion sheet on which light diffusing particles is applied can not resolve the problem of the lamp image.
Thus, light diffusion plates containing the light diffusing particles are popularly used. For example, a fine particles-containing light diffusion plate 20 is provided on the front surface of the light source 1 behind which the reflector 2 is disposed as FIG. 23. The method of using a fine particles-containing light diffusion plate composed by combining a base resin such as methacryl resin, polycarbonate resin, styrenic resin and vinyl chloride resin with fine inorganic particles or bridging fine organic particles as the light diffusing particles was considered. (c.f. Patent document 2)
However, in the method of using the light diffusing particles, light is absorbed into the light diffusing particles and diffused into the unnecessary direction so that the efficiency of using light is reduced. Therefore it is not preferable in view of energy saving. Additionally, the lamp image can be reduced by disposing adjacently a number of light sources, meanwhile power consumption is increased undesirably.
On the other hand, the method of eliminating the lamp image by forming the reflector in a unique shape is proposed. (c.f. Patent document 3). Where, it is required to align the reflector with the light source and the thinning is prevented due to the reflector shape. Accordingly, the method is not appropriate.
Additionally, the method of providing the reflective member opposite to the light source (c.f. Patent document 4), and the method of disposing light beam direction sensing element such as a Fresnel lens (c.f. Patent document 5) are proposed. Where, it is required to precisely align the member with the light source as well as the method described in the Patent document 3. Therefore, the problem that it is nonproductive occurs.
Further, the method of diffusing light using prism sheets is proposed. (c.f. Patent document 6) Where, those sheets can only split the light from the normal light source into two similar images. Therefore, the sufficient luminance uniformity can not be obtained.
As for the large lighting system, the thinness is not strictly required in comparison with cellular phones and mobile computers. Therefore, the distance between the light source and the light diffusion plate is reduced, and the number of the optical film is reduced so that it can deal with the problems.
Additionally, in order to save energy, the efficiency of using light should be improved. The direct type can increase the number of the light source and obtain high luminance. However, the large number of light diffusing particles to eliminate the lamp image, which significantly reduce the efficiency of using light should be avoided to be used in view of energy saving.